Atmospheric Pollution from the Internal Combustion Engine in the Urban Environment

The first four-stroke internal combustion engine was built by Nikolaus August Otto in May 1876. Otto's design was based partly on an earlier gas engine invented by Etienne Lenoir. The four-stroke cycle had, however, been patented earlier by Aphonse Beau de Rochas in 1862. Though Otto was the first to actually build a working model, in 1886 his patent was revoked.

Since then, the four-stroke internal combustion engine (and its two-stroke relative) has been used in automobiles, motorbikes, motorboats and many other vehicles. It has, in many ways, changed the face of the world. (Other internal combustion engine types are diesel, gas turbine and jet engines.)

This Entry focuses on the local effects of chemical pollution from exhaust fumes. While internal combustion engines have many roles and appear in many guises, the application which has the most effect on the urban environment is almost certainly that of transport, particularly private transport. This Entry, therefore, will be on the subject of car exhaust.

A car's exhaust is by no means a simple thing to study. Motor fuel is obtained from crude oil produced predominantly by the decay of marine organisms1. It therefore contains the elements carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), along with some amounts of sulphur (S), all of which are found in protein. Hence, motor fuel contains hydrocarbons and organic compounds containing nitrogen and sulphur. When these are burned in air the products are water, carbon di- and monoxide and oxides of nitrogen. Nitrogen gas in the atmosphere can also react with oxygen at the high temperatures in the combustion chamber to form oxides.

Also, while the internal combustion engine is more efficient than the external, it is by no means as efficient as we could wish. Much of the hydrocarbon fuel passes through the process unconsumed and is expelled into the atmosphere along with other exhaust fumes.

According to the Ohio EPA, 'The average American automobile emits its own weight in pollutants into the atmosphere each year'.

Pollutants from Car Exhaust

CO2 — carbon dioxide. This gas is naturally present in the atmosphere at low concentration (approximately 0.035%). It absorbs infrared energy and is thus a greenhouse gas (a contributor to global warming). Concentrations of CO2 in the earth's atmosphere appear to be increasing. This could have a substantial effect on the climate. The internal combustion engine contributes to the increased concentrations of CO2 in the atmosphere. The effect of carbon dioxide, however, is felt worldwide. It does not have a great impact on the immediate urban environment2. Nor are car engines the greatest producers of this pollutant.

CO — carbon monoxide. The main source of CO in cities is the internal combustion engine, where it is produced by incomplete combustion. Anthropogenic sources account for approximately 6% of the 0.1 ppm concentration of CO in the earth's atmosphere globally. In an urban area, the concentration (and the percentage anthropogenic contribution) can be much higher. During a city rush hour, for example, concentrations of CO can reach 50 or even 100 ppm, which greatly exceeds the safe level. CO is highly toxic: it binds to haemoglobin more strongly than oxygen does, thus reducing the capacity of the haemoglobin to carry oxygen to the cells of the body. CO also has the nasty habit of sticking to haemoglobin and not coming off. This means that a fairly small amount of it can do a lot of damage.

CO can be oxidised to the far less harmful CO2 if there is enough O2 available. At higher air-fuel ratios the level of CO emission goes down. The fuel has undergone complete, or more nearly complete, combustion. CO can also be oxidised to CO2 in a catalytic convertor.

NOx — oxides of nitrogen. While some nitrogen may be present in the fuel (as mentioned earlier), most oxides of nitrogen are produced when elemental nitrogen (N2) in the air3 is broken down and oxidised at high temperatures (approximately 1000 K or greater) and pressures within the internal combustion engine. Nitrogen monoxide (NO) is produced in higher concentration than nitrogen dioxide (NO2) but the two species are in any case interconvertible by means of photochemical interactions. Other oxides of nitrogen, such as N2O4, may occur; but are more rare. Because hydrocarbons compete with nitrogen for oxygen, NO is formed to a greater extent in cars with a 'lean mixture', that is, a low fuel-air ratio.

NO and NO2 are toxic species. Oxides of nitrogen also play a major role in the formation of photochemical smog, which is discussed below.

HC — hydrocarbons. 'Much of the hydrocarbon fuel passes through the process unconsumed and is expelled into the atmosphere along with other exhaust fumes'. This remark was made earlier in passing. Fuel close to the wall of the combustion chamber may be quenched by the relative coolness of that area and not be burned. If the engine is poorly designed or is not in proper working order the proportion of unburned fuel rises. Hydrocarbons are also released to the atmosphere by evaporation from fuel tanks. Hydrocarbons can be dangerous to human health and are also part of the makeup and cause of photochemical smog, which is discussed below.

C6H6 — Benzene and its derivatives. Benzene is, of course, a hydrocarbon, but is sufficiently different from straight-chain hydrocarbons to merit a separate discussion. The six carbons in benzene form a regular hexagon, with one hydrogen attached to each carbon and sticking out (away from the centre of the hexagon). All 12 atoms lie on one plane. This structure is extremely stable — stable enough for a large proportion of the benzene in fuel to pass unchanged through the combustion process. There is quite a lot of benzene in fuel. It acts as an anti-knock agent, making cars run more smoothly. Since the abolition of lead additives as anti-knock agents, the levels of benzene and benzene-related compounds in car fuel have increased.

Benzene may be present in fuel as itself or in a modified form, where one or more hydrogens have been removed to form a phenyl ring and other things have been attached in their places. A phenyl ring (Ph) has the formula C6H5—; it may be found attached to a long straight-chained hydrocarbon. During combustion, a molecule with this structure may be broken down entirely or release benzene or a derivative.

Benzene (C6H6), and also many of its derivatives such as toluene (PhCH3) and phenol (PhOH), is carcinogenic (the level of toxicity varies). Benzene vapours are therefore a danger. It has been suggested that benzene is more dangerous to filling station attendants than to the general public in the streets as the concentration of benzene will be higher in the raw fuel than in the combustion products. (Some fuel evaporates from fuel tanks; some is spilled while tanks are being filled and thereafter evaporates from the ground. The atmosphere of filling stations will always contain a high concentration of hydrocarbons, including phenyl rings.)

Benzene rings may also be fused to form polycyclic aromatic hydrocarbons. These are particularly prevalent in diesel exhaust.

SO2 — sulphur dioxide. Fossil fuels are derived from once-living organisms. Some sulphur occurs in protein and will still be present in the fuel. Under combustion this sulphur reacts with oxygen to form sulphur di- and trioxide. Sulphur is more prevalent in solid fuel (such as coal) than in liquid, but some sulphur dioxide emission does occur from cars. SO2 and SO3 are acidic pollutants which dissolve in moisture in the atmosphere to form sulphurous and sulphuric acids (H2SO3 and H2SO4), which are components of 'acid rain'. These corrode metal surfaces and weather limestone buildings.

Acid rain also mobilises toxic aluminium ions (Al3+) in the soil, washing them out into streams and ponds. This causes a sticky mucus to accumulate in the gills of fish and eventually kills them. Trees and other plants which absorb Al3+ ions will be damaged.

In humans, sulphur dioxide irritates the eyes, the mucous membranes and the respiratory tract, along with the skin in general. SO2 also has the effect of slowing down the movements of the cilia (the hairs in the trachea which act to prevent dust entering the lungs), thus exacerbating the irritation caused by allowing more pollutant to access the respiratory system. SO2 is a component of 'classical' smog (as exemplified by London in the 1950s4).

PM10s - particles micro-particulate, 10 microns. These are ultra-fine particles which are less than one-hundredth of a millimetre across. Thus they are too small to settle or be dispersed by rain. These particles absorb acidic gases which are also present in exhaust fumes and, when inhaled, penetrate into the microscopic air sacs of the lungs (alveoli). Scavenging white blood cells (macrophages) are overwhelmed by these particles, and release a stream of chemicals that trigger an inflammatory reaction in the lungs, and increase the stickiness of red blood cells, thus increasing the likelihood of blood clots. The main victims of this type of pollution are the elderly, smokers, and those suffering from chest complaints, heart conditions and, to a lesser extent, asthma.

It is considered that PM10s may be the most important and dangerous component of vehicle pollution. These particles can drift for miles, and accumulate inside buildings. The major source of PM10s in urban air is motor vehicles, particularly diesel engines. Even unpolluted air can contain up to 30 micrograms per cubic metre of PM10s, but during smoggy conditions in British cities, concentrations can rise above 100 micrograms per cubic metre. Epidemiological studies suggest that for each 10 microgram/cubic metre rise, the number of daily deaths, hospital admissions and asthma attacks increase by at least 1%.

Photochemical Smog

Reactive pollutant hydrocarbons in the presence of NOx and under certain atmospheric conditions can produce a brown haze known as photochemical smog. It is given this name because it is formed by photochemical reactions (that is, reactions catalysed by light) between NOx and hydrocarbons. Normally, any combustion process hot enough to produce oxides of nitrogen (that is, with enough energy to break the triple bond in N2) will also fully oxidise hydrocarbons, breaking them down to water and carbon dioxide. Conversely, any combustion process which does not fully consume hydrocarbons, allowing some to pass to the atmosphere, should not possess sufficient energy to oxidise nitrogen. In the internal combustion engine, however, there is a combination of extremely high temperatures and a shortage of oxygen. The two essential ingredients for the formation of photochemical smog are produced together.

When high levels of NO2 are present in the atmosphere it can undergo the reactions NO2 --> NO. + O. O. + O2 --> O3 O3 + NO. --> O2 + NO2 in a cycle5. The relative concentrations of NO and NO2 are important. NO2 drives the formation of ozone; NO destroys ozone. When the ratio of NO2 to NO is greater than three, ozone concentrations will rise, possibly to dangerous levels. (While ozone plays an essential protective role in the stratosphere, it is viewed as a dangerous pollutant in the troposphere6. It is highly oxidising and causes irritation to the eyes.)

Methane, CH4, is less reactive than are other hydrocarbons, but as it is a simple compound some of its reactions are shown here. Reactions of other hydrocarbons would be similar. CH4 + O. --> H3C. + HO.H3C. + O2 --> H3COO.CH4 + HO. --> H3C. + H2O
H3COO. + NO --> H3CO. + NO2H3COO. + NO2 --> CH3OONO2 The initial oxygen radical may be produced by photodissociation of nitrogen dioxide, which is regenerated further on in the series. The product of the last reaction mentioned is peroxyacetylnitrate, a very strong oxidant. The chemistry of photochemical smog is highly complex. There will be many free radicals and highly reactive organic species (including aldehydes and ketones) present. There will also be many polycyclic aromatic hydrocarbons and many other pollutants in the atmosphere. The five reactions shown above are by no means a full summary of what is going on in this cocktail.

These reactions are photochemical. This means that they require energy from light of a certain wavelength. Photochemical smog is most common on windless sunny days when the ingredients are not dispersed and there is plenty of light energy available to power the reaction.

Again, according to the Ohio EPA, emissions from car exhaust account for about 60% of ozone smog in cities.

Photochemical smog is characterised by the presence of particulate matter (which creates a sort of haze), oxidants such as ozone, and noxious organic species such as aldehydes.

Suggested Solutions

Catalytic Converters. Most modern cars contain catalytic converters. In these, exhaust fumes and added air pass over a catalyst where they are broken down to less harmful products.

Drive less. It may be fairly obvious, but people are very reluctant to do it. Pedestrian zones should be increased; cycling should be encouraged. Many techniques have been discussed, including 'Park & Ride' schemes. The use of public transport and of carpools can be of help. According to Alpha Nutrition, 'Driving a car is the most polluting act an average citizen commits'.

Use cleaner engines. Many modern cars are designed to be more fuel-efficient and less pollutant. (These vehicles are usually at their best in cities. Pick the car that's best for the way you use it.) Less pollutant cars may cost a little more, but they'll pay for themselves in the feeling of smugness they give you! On a more serious note, some governments are considering giving grants to make this type of car cheaper to purchase. Also, of course, if they use less fuel they are cheaper to run. There is less environmental damage caused by fuel production and delivery.

Drive hybrid vehicles. Whether a hybrid engine is more environmentally sound than a normal engine depends on how the car is used. For city stop-start driving, they're usually better. Consider how you use your motor, do some research, and purchase accordingly.

Keep your car in good working order. An engine will be more environmentally friendly when it runs more efficiently. According to the Ohio EPA, more than 80% of emissions come from less than 30% of cars. This is related to the point above.

Use smaller cars. There are some good reasons for driving massive vehicles. They have their place, certainly. But do any of these reasons apply to you? A heavier vehicle will naturally consume more fuel, producing a large proportion of pollutants.

1There is some evidence that crude oil is also produced by abiotic (non-biological) mechanisms.2Though it must be said that raised sea levels caused by global warming would not be pleasant for many cities.3Air is 78% elemental nitrogen.4The London Met Office estimates that approximately 4,000 deaths occurred in London in 1952 as a result of respiratory illness caused by the severe smog that year.5The raised dot indicates the presence of an unpaired electron on the chemical species. Species with unpaired electrons are called 'free radicals' and are very unstable (which is another way of saying that they are highly reactive).6That's the bit of the atmosphere closest to the earth, the bit we live in.

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